The present invention relates to the control of engine valves, such as intake and exhaust valves. In particular, the invention relates to methods and apparatus for controlling valve seating velocity.
Engine combustion chamber valves, such as intake and exhaust valves, are almost universally of a poppet type. These engine valves are typically spring loaded toward a valve closed position. In many internal combustion engines the engine cylinder intake and exhaust valves may be opened and closed by fixed profile cams in the engine, and more specifically by one or more fixed lobes which may be an integral part of each of the cams. The use of fixed profile cams makes it difficult to adjust the timings and/or amounts of engine valve lift to optimize valve opening times and lift for various engine operating conditions, such as different engine speeds.
A variety of systems exist to regulate the timing of engine valve opening by controlling the hydraulic pressure that acts on a slave piston which actuates the engine valve. These systems include xe2x80x9ccommon railxe2x80x9d systems in which a solenoid control valve opens a path from a source of high pressure fluid to the top of the slave piston at precise times. One such common rail system is described in Cosma et al., U.S. Pat. No. 5,619,964, assigned to the assignee of the present application.
Another method of adjusting valve timing and lift, given a fixed cam profile, has been to incorporate a xe2x80x9clost motionxe2x80x9d device in the valve train linkage between the valve and the cam. Lost motion is the term applied to a class of technical solutions for modifying the valve motion proscribed by a cam profile with a variable length mechanical, hydraulic, or other linkage means. In a lost motion system, a cam lobe may provide the xe2x80x9cmaximumxe2x80x9d (longest dwell and greatest lift) motion needed over a full range of engine operating conditions. A variable length system may then be included in the valve train linkage, intermediate of the valve to be opened and the cam providing the maximum motion, to subtract or lose part or all of the motion imparted by the cam to the valve.
This variable length system (or lost motion system) may, when expanded fully, transmit all of the cam motion to the valve, and when contracted fully, transmit none or a minimum amount of the cam motion to the valve. An example of such a system and method is provided in Hu, U.S. Pat. Nos. 5,537,976 and 5,680,841, which are assigned to the same assignee as the present application and which are incorporated herein by reference.
In the lost motion system of U.S. Pat. No. 5,680,841, an engine cam shaft may actuate a master piston which displaces fluid from its hydraulic chamber into a hydraulic chamber of a slave piston. The slave piston in turn acts on the engine valve to open it. The lost motion system may include a solenoid valve and a check valve in communication with the hydraulic circuit including the chambers of the master and slave pistons. The solenoid valve may be maintained in a closed position in order to retain hydraulic fluid in the circuit. As long as the solenoid valve remains closed, the slave piston and the engine valve respond directly to the motion of the master piston, which in turn displaces hydraulic fluid in direct response to the motion of a cam. When the solenoid is opened temporarily, the circuit may partially drain, and part or all of the hydraulic pressure generated by the master piston may be absorbed by the circuit rather than be applied to displace the slave piston.
Another example of an engine valve actuator is disclosed in U.S. Pat. No. 5,186,141, xe2x80x9cengine Brake Timing Control Mechanism,xe2x80x9d issued to D. Custer on Feb. 16, 1993 (the xe2x80x9c""141 patentxe2x80x9d), incorporated by reference herein. The actuator disclosed in the ""141 patent does not provide for engine valve seating control, although it could benefit from such control.
Engine valves are required to open and close very quickly, therefore the valve spring is typically very stiff. When the valve closes, it may impact the valve seat with such force that it eventually erodes the valve or the valve seat, or even cracks or breaks the valve. In mechanical valve actuation systems that use a valve lifter to follow a cam profile, the cam lobe shape provides built-in valve-closing velocity control. In common rail hydraulically actuated valve assemblies, however, there is no cam to self-dampen the closing velocity of an engine valve. Furthermore, in some lost motion applications the engine valve needs to be closed at an earlier time than that provided by the cam profile. This earlier closing may be carried out by rapidly releasing hydraulic fluid to an accumulator in the lost motion system. In hydraulic lost motion systems, a rapid draining of fluid from the slave piston may allow an engine valve to xe2x80x9cfree fallxe2x80x9d and seat with an unacceptably high velocity. Free fall results when the rate of closing the engine valve is governed by the hydraulic fluid flow to the accumulator instead of by the fixed cam profile. Engine valve seating control may also be required for applications (e.g. centered lift) in which the engine valve seating occurs on a high velocity region of the cam. Electromagnetic valve actuation may also require valve seating control.
As a result of the foregoing there is a need to limit valve seating velocities. The need for limited valve seating velocities conflicts with the need for rapid valve opening rates. Some attempts have been made to solve the problem by providing separate fill and drain ports for slave pistons. U.S. Pat. No. 5,577,468 discloses one system for limiting valve seating velocity. Existing methods for controlling engine valve seating velocity may be costly, inaccurate, and cause excessive valve closing variations. Existing systems also fail to accommodate the need for adjustments due to variations in engine valve lash between cylinders.
Applicants approached the valve seating challenge with the understanding that valve seating velocity should be less than approximately 15 in/sec (0.38 m/sec). Absent steps to control valve seating velocity, the valves could seat at a velocity that is an order of magnitude greater. Applicants also determined that valve seating control preferably should be designed to function when the closing valve gets within 0.5 to 0.75 mm of the valve seat. The combination of valve thermal growth, valve wear, and tolerance stack-up can exceed 0.75 mm, resulting in the complete absence of seating velocity control or in an exceedingly long seating event if measures are not taken to adjust the lash of the valve seating control to account for such variations. It is also assumed that, preferably, valve seating control should not significantly reduce initial engine valve opening rate, and valve seating control should be capable of operating over a wide range of valve closing velocities and oil viscosities.
Valve catch devices used to control valve seating velocity may use hydraulic fluid flow restriction to produce pressure that acts on an area of the slave piston to develop a force to slow the slave piston and reduce seating velocity. The area on which the pressure acts may be very small in such devices which in turn requires that the pressure opposing the valve return spring be high, and the controlling flow rate be low. Low controlling flow rates result in an increased sensitivity to leakage and manufacturing tolerances. In addition, these devices may restrict the hydraulic fluid flow that produces valve opening.
A known valve catch (seating) system developed to provide valve seating control is disclosed in co-pending U.S. patent application Ser. No. 09/383,987, filed Aug. 26, 1999, hereby incorporated by reference and which is shown as system 100 in FIG. 1. The system 100 includes a slave piston 120 disposed within an actuator housing 110. The slave piston 120 is slidable within the housing 110 so that it may open an engine valve (not shown) below it. A screw body 130 extends through the top of the housing 110 and abuts against the slave piston 120 when the latter is in a resting position (i.e. engine valve closed). A plunger 140 is disposed within the screw body 130 and is biased towards the slave piston 120 by a spring 160. The screw body 130 may be twisted into and out of the housing 110 to manually adjust engine valve lash.
The plunger 140 serves to selectively limit valve seating velocity as the slave piston 120 approaches its home position (engine valve closed), thereby allowing the engine valve to close more gently than it otherwise might. The plunger 140 is mechanically limited from extending beyond the screw body 130 by more than a preset distance, thus allowing the slave piston 120 to return rapidly until contacting the plunger.
The system 100 operates under the influence of hydraulic fluid provided through a passage 150 in the housing 110. Preferably, the hydraulic fluid provided by the passage 150 is high pressure. During the downward (valve opening) displacement of the slave piston 120, hydraulic fluid flows through the passage 150 in the housing 110 and through the passages in the slave piston so that the slave piston is forced downward against the engine valve. During the upward (valve closing) displacement of the slave piston 120, the hydraulic fluid flows back through the passages in the slave piston 120 and out of the passage 150 in the housing 110. As the slave piston 120 approaches its home position, it forms a seal with the plunger 140. The seal between the plunger 140 and the slave piston 120 results in the building of hydraulic pressure in the space between the slave piston and the end wall of the housing 110 as the slave piston progresses towards its home position. The building hydraulic pressure opposes the upward motion of the slave piston 120, thereby slowing the slave piston and assisting in seating the engine valve.
While the valve catch system 100 shown in FIG. 1, which works on slave piston pressure, has achieved acceptable valve seating velocity over a wide range of engine speeds and oil temperatures, improvements are still needed. For example, the valve catch system 100 tends to hold the engine valve open longer than is desirable for optimum engine breathing at high engine speeds. The system is also prone to reduce valve velocity to nearly zero prior to seating and thereafter accelerate the valve so that it seats at an unacceptable velocity. This type of valve catch system also may require a complicated slave piston design, which increases high-pressure volume, increases the length and flow resistance of the fluid path between the slave piston and the passages leading to the master piston, trigger valve, or plenum, and increases the required slave piston height and weight. Increased high-pressure volume may be detrimental to compliance. Increased flow path length and flow resistance provide increased pressure drop and therefore increased parasitic power and oil cooling load. Additionally, increased pressure drop may make it difficult to maintain master piston pressure greater than ambient during periods of decreasing cam displacement during high engine speed, which may allow air bubbles to form in the oil.
A second valve catch system 200 is disclosed in the co-pending 09/383,987 application referenced above, and is shown in FIG. 2. The valve catch system 200 works on valve catch plenum pressure, and is considered to have lower parasitic loss than the system shown in FIG. 1. The system 200 includes a slave piston 220 disposed within an actuator housing 210. The slave piston 220 is slidable within the housing 210 so that it may open an engine valve (not shown) below it. A screw body 230 extends through the top of the housing 210 and abuts against the slave piston 220 when the latter is in a resting position (i.e. engine valve closed). A plunger 240 is disposed within the screw body 230 and biased towards the slave piston 220 by a spring 260. The screw body 230 may be twisted into and out of the housing 210 to adjust engine valve lash. A fluid passage 250 through the housing 210 leads to a high pressure hydraulic source such as a master piston (not shown) and/or a trigger valve (not shown).
The system 200 operates similarly to the system 100 shown in FIG. 1, except that the hydraulic pressure that opposes the upward movement of the slave piston 220 is built inside the screw body 230. Although performance may be improved using the system 200, compliance difficulties may still be encountered due to the high pressures required and the increased compliance associated with the smaller area of plunger 240.
In view of the foregoing there is a need for a system for valve seating control that operates well in a high pressure regime requiring fine control of hydraulic fluid flow through the system. There is also a need for a system that does not adversely effect hydraulic fluid flow for valve opening and which is less susceptible to leakage sensitivity. In particular, there is a need for valve seating that is improved by a flow control that becomes more restrictive as the valve approaches the seat.
There is also a need for a valve catch that adjusts for lash differences between the engine valve and the valve catch. Although most variable valve actuation (VVA) systems are inherently self lash adjusting, valve seating control is not. Systems that do not need manual adjustment, either initially, or as the system ages, are desirable. Previous valve seating control mechanisms have required a manual lash adjustment or a separate set of lash adjustment hardware. The design of a conventional hydraulic lash adjustor capable of transmitting compression-release braking loads would be challenging due to structural and compliance requirements.
Unlike the valve catch systems 100 and 200 shown in FIGS. 1 and 2, the various valve catch embodiments of the present invention include a variable area orifice in the system plunger. Accordingly, the various valve catch embodiments of the invention may have reduced parasitic power loss and consequently reduced VVA housing cooling load, and reduced slave piston length and weight as compared with previous valve catch systems. The valve catch embodiments of the present invention may also experience reduced peak valve catch pressure as compared with the previous valve catch systems. Furthermore, the variable flow restriction design of the valve catch embodiments of the present invention is expected to be more robust than the constant flow restriction design in terms of engine valve velocity control at the point of valve catch engagement, and in terms of oil temperature and aeration control. Variable flow restriction may allow the displacement at the point of valve catch/slave piston engagement to be reduced, so that the valve catch has less undesired effect on the breathing of the engine.
The present invention meets the aforementioned needs and provides other benefits as well. The claimed invention provides acceptable engine valve seating velocity in a VVA system, such as a lost motion or common rail system. For a lost motion VVA system, engine valve seating control is provided for early engine valve closing, where the rate of closing is governed by the hydraulic flow from the slave piston to the accumulator as opposed to a cam profile. Engine valve seating control also may be provided for a high velocity region of the cam and/or for common rail VVA designs.
The valve seating velocity control provided by this invention also may be applied to camless variable valve actuation designs in which the engine valve is not spring loaded toward a valve-closed position. One example is the electromagnetic concept (Aura, FEV, BMW, Daimler Benz, Siemens) in which there are opposing springs acting in both the valve closed and valve open directions, in order to create an oscillating spring-mass system, and two solenoids, which latch the valve in either the closed or full-open position. In this system, valve seating velocity control could be provided by precisely controlling the current to the solenoids; however, in practice, a separate valve seating control device may be required to assure acceptable valve seating under all conditions. Another example is the electrohydraulic common rail concept (Ford) in which there are no valve springs and two high-speed solenoid valves are used to alternately connect a source of high-pressure hydraulic fluid and drain to either side of a piston connected to the engine valve. In this system, valve seating velocity control could be provided by precisely controlling the timing of the high-speed solenoid valves; however, in practice, a separate valve seating control device may be required to assure acceptable valve seating under all conditions.
It is therefore an object of the present invention to provide a method and system for controlling valve seating.
It is a further object of the present invention to provide a method and system for controlling valve seating that operates well under high hydraulic pressure.
It is another object of the present invention to provide a method and system for seating an engine valve with fine control over valve seating velocity.
It is yet a further object of the present invention to provide a method and system for controlling valve seating that is less likely to adversely affect hydraulic fluid flow for valve opening.
It is still another object of the present invention to provide a method and system for controlling valve seating that progressively restricts hydraulic fluid flow as the valve approaches its seat.
It still yet another object of the present invention to provide a method and system for controlling valve seating velocity that is self lah adjusting for valve thermal growth, valve wear, and tolerance stack-up.
It is yet another object of the present invention to provide a method and system for valve seating control that provides a nearly constant deceleration of the valve before seating.
It is still yet a further object of the present invention to provide a method and system for valve seating control that provides acceptable seating velocity during early valve closing events.
It is still another object of the present invention to provide a method and system for valve seating control that provides acceptable seating velocity during centered lift events when the valve seats on a high velocity section of the cam.
It is still another object of the present invention to provide a method and system for valve seating control that reduces the volume of hydraulic fluid in the master-slave piston circuit in order to reduce system compliance.
It is a further object of the present invention to provide a method and system for valve seating control with reduced parasitic loss and consequently reduced cooling requirements.
It is another object of the present invention to provide a method and system for valve seating control with improved hydraulic fluid aeration characteristics.
It is still another object of the present invention to provide a method and system for valve seating control that utilizes a slave piston of reduced length and weight as compared to previous systems.
It is still a further object of the present invention to provide a method and system for valve seating control of relatively simple and low cost design.
It is still a further object of the present invention to provide a method and system for reliable valve seating.
It is another object of the present invention to provide a method and system for controlled seating velocity over a wide range of valve closing velocities.
It is still another object of the present invention to provide a method and system for controlled seating velocity over a wide range of oil viscosities.
It is still a further object of the present invention to provide a method and system for valve seating control that does not require closely held concentricity of the control elements.
It is still a further object of the present invention to provide a method and system for valve seating control that includes a means to dissipate the heat generated during valve seating.
Additional objects and advantages of the invention are set forth, in part, in the description which follows and, in part, will be apparent to one of ordinary skill in the art from the description and/or from the practice of the invention.
It is a further object of the present invention to provide a method and system for valve seating control that does not require closely held concentricity of the control elements.
It is a further object of the present invention to provide a method and system for valve seating control that includes a means to dissipate the heat generated during valve seating.
In response to the foregoing challenges, Applicants have developed an engine valve seating system having a piston adapted to be bi-directionally displaced in response to the filling and draining of hydraulic fluid from an hydraulic chamber in communication with the piston, the system comprising means for guiding hydraulic fluid from the chamber during draining; and means for throttling hydraulic fluid flow through the guiding means at a preselected rate in response to a change in position of the guiding means relative to the throttling means during draining.
Applicants have additionally developed a system for controlling seating velocity of an internal combustion engine valve comprising a housing having a bore formed therein for receipt of a piston; a piston positioned in and adapted for bi-directional displacement in the bore; an hydraulic chamber defined by an end of the piston; a piston stop extending into the chamber; and a disk having at least a central opening, the disk positioned in the chamber and being adapted to cooperate with the piston stop to control valve seating velocity.
Applicants have additionally developed a system for controlling seating velocity of an internal combustion engine valve comprising a housing having a bore formed therein for receipt of a piston, and a recess formed in an end wall of the bore; a recess shoulder formed along the intersection of the recess and the bore; a piston positioned in and adapted for bi-directional displacement in the bore; an hydraulic chamber defined by the bore end wall and the piston; means for providing hydraulic fluid flow to and from the chamber; a disk having at least a central opening, said disk positioned between the piston and the bore end wall; a spring adapted to bias the disk against the recess shoulder when the piston is in a retracted position; and an elongated stop having a fluted end extending from the piston, through the chamber, through the disk, and into the recess, wherein a minimized hydraulic passage is formed between the disk and the elongated stop when the piston is in the retracted position.
Applicants have additionally developed a system for controlling seating velocity of an internal combustion engine valve comprising a housing having a bore formed therein for receipt of a piston; a piston positioned in and adapted for bi-directional displacement in the bore, the piston having a recess formed in an upper end thereof; an hydraulic chamber defined by an end wall of the bore and the upper end of the piston; a recess shoulder formed along the intersection of the recess and the chamber; a disk having a central opening, the disk positioned between the piston and the bore end wall; a spring adapted to bias the disk against the recess shoulder when the piston is in a retracted position; and an elongated stop having a fluted end extending from the bore end wall, through the chamber, through the disk, and into the recess, wherein a minimized hydraulic passage is formed between the disk and the elongated stop when the piston is in the retracted position.
Applicants have also developed a method of controlling the seating velocity of an engine valve comprising the steps of filling a fluid chamber responsive to an opening motion of the engine valve; expulsing fluid from the fluid chamber responsive to a closing motion of the engine valve; and progressively throttling the expulsion of fluid from the fluid chamber during at least a portion of the engine valve closing motion.
Applicants have additionally developed a method of controlling the seating velocity of an engine valve and providing automatic lash take up, the said method comprising the steps of providing leakage filling of a first fluid chamber to automatically take up lash; filling a fluid chamber responsive to an opening motion of the engine valve; expulsing fluid from the fluid chamber responsive to a closing motion of the engine valve; and progressively throttling the expulsion of fluid from the fluid chamber during at least a portion of the engine valve closing motion.